The response of mononuclear phagocytes to various pathologic states will be examined by electron microscopic and cytochemical techniques. Emphasis will be placed on determining the fate and function of the two differing kinds of stored secretory granules of the blood monocyte during the course of host defense reactions. More precisely, the aims of this research are: 1. To investigate early stages of differentiation of the "immune" or activated macrophage. 2. To analyze lysosomal enzyme delivery to phagosomes and "immune" or activated macrophages. 3. To assess the function of the mononuclear phagocyte in chronic inflammation and the stages of differentiation epithelioid and giant cells. 4. To evaluate the function of mononuclear phagocytes during induction of the humoral immune response. 5. To survey the lysosomal pathways of mononuclear phagocytes in various tissues.